Three studies on retina are in progress. cGMP levels have been measured in the laters of frog retina as a function of light, using both a tungsten lamp and fiber optics as light sources, and times ranging from 0.1 sec to 2 hours of light. There was a drop in cGMP by 0.1 sec followed by further and larger drops at 2 sec and 2 min. In addition, there was a high level of variability in both dark animals as well as those exposed to short periods (0.1, 1 and 2 sec) of light. This variability disappeared after longer (2 minute) exposures. High energy phosphate compounds are being measured in retinas of dogs bred to develop a progressive retinal dystrophy. Eyes from controls, carriers and diseased animals of varying chronological age are sectioned, freeze-dried, and the retinas dissected into 8 layers plus tapetum. They are then analyzed by the oil-well technique for ATP plus ADP to see if there are changes in metabolite levels with either age or disease. A new phosphodiesterase has been isolated from monkey and bovine retinas. It is in high concentration in the interphotoreceptor matrix between outer segments and pigment epithelial processes. It has a subunit Mr=43 K but has a high Mr native configuration. The Km for cGMP is about 35 uM while that for cAMP is 2.3 mM. It occurs tightly bound to an inhibitory protein which is heat stable.